1. Field of the Invention
The present invention relates to an ultrasonic diagnosis apparatus, and more particularly to an apparatus for performing diagnosis based on secondary harmonic information generated by a living body.
2. Description of the Related Art
In recent years, a physical quantity known as the xe2x80x9cnon-linear parameterxe2x80x9d has gained attention in the field of observation and diagnosis of the interior of a living body using ultrasound. This non-linear parameter represents the degree of non-linear interaction of an acoustic wave with a medium, such as body tissue or an ultrasonic contrast agent comprising microbubbles. It is presumed that, based on the non-linear parameter, information such as the water content of the body tissue can be obtained, and the contrasting effect of the ultrasonic contrast agent can be achieved.
Based on the phenomenon that sound velocity increases as sound pressure becomes higher, distortion generated in an ultrasound propagating through a body is induced by the acoustic non-linearity of the body tissue, thereby accumulatively producing secondary harmonic component. In addition, secondary harmonic echo is generated based on the non-linear vibration characteristic of the ultrasonic contrast agent. It is known that, when distortion of an ultrasound is not large, the amplitude of the produced secondary harmonic is proportional to the intensity (square of the amplitude) of the fundamental. Further, the amplitude of the secondary harmonic depends on the non-linear parameter of the medium.
In view of the above, transmission of the fundamental of the center frequency f0 to the medium, and then defining the non-linear parameter based on the secondary harmonic component of the frequency 2f0 included in the received echo, has been conventionally proposed.
When the intensity of the transmitted ultrasound is P0(f0), the distance from the probe is z, and the frequency-dependent attenuation coefficient xcex1(f,z) is a function of the frequency f and the distance z, the amplitude A2(z) of the secondary harmonic of the echo signal received from scatterers having a backscattering characteristic xcex3(f,z) can be represented by the following equation.
A2(z)=P0(f0)xc2x7exp (xe2x88x922∫xcex1(f0,z)dz)xc2x7exp (xe2x88x92∫xcex1(2f0,z)dz)xc2x7xcex3(2f0,z)xc2x7∫h(z)dzxe2x80x83xe2x80x83(1)
The factor xe2x80x9cP0(f0)xc2x7exp(xe2x88x922∫ xcex1(f0 ,z)dz)xe2x80x9d on the right side of the equation (1) represents the intensity of the transmitted fundamental which has been attenuated by the distance z. The factor xe2x80x9c2xe2x80x9d in the exponent is derived from the fact that the distortion is proportional to the square of the amplitude (intensity) of the fundamental. The next factor xe2x80x9cexp(xe2x88x92∫ xcex1(2f0,z)dz)xe2x80x9d represents the attenuation to which the secondary harmonic scattered wave was subjected in the distance z until reaching the probe. The factor xe2x80x9ch(z)xe2x80x9d in the final integrating factor is a term reflecting the non-linear parameter (B/A) of the medium in the distance z. This term can be represented by the following equation including the sound velocity C0 and the density xcfx810 of the medium during equilibrium. The value of B/A is known to be about 5 to 11 in a body tissue, while much greater in a microbubble medium.
h=(B/A+2)xc2x72xcfx80f0/(4xcfx810C03)xe2x80x83xe2x80x83(2)
In the body tissue, the final factor is obtained by integrating h(z) with respect to the distance z. This factor is provided corresponding to the accumulation of the secondary harmonic information generated along the propagation of the transmitted ultrasound. It is assumed, however, that secondary harmonic component generated by the non-linear vibration of the ultrasonic contrast agent does not accumulate, and that h(z) is not integrated with respect to the distance.
As is understood from the equation (1), secondary harmonic component included in an echo signal includes a factor dependent on the frequency-dependent attenuation characteristic xcex1 and the backscattering characteristic xcex3. Such secondary harmonic information, can therefore not be directly used as an evaluation value of the non-linear parameter.
Under the above circumstances, Akiyama et al. proposed, in Japanese Journal of Applied Physics, vol.30, supplement 30-1 (1991), re-transmitting the fundamental toward the same location in the body with the center frequency of the transmitted fundamental set to 2f0, and removing the influence of the attenuating characteristic and the scattering characteristic by making use of the phenomenon that the echo signal is subjected to the same attenuating and scattering characteristics as those of the secondary harmonic A2(z).
Specifically, in the system proposed by Akiyama, the influence of the attenuating and scattering characteristics is removed by the following processing. Assuming that the amplitude of the transmitted fundamental having the center frequency of 2f0 is A0(2f0), the fundamental amplitude AII(z) of the received echo signal is represented by the following equation.
AII(z)=A0(2f0)xc2x7exp (xe2x88x922∫xcex1(2f0,z)dz)xc2x7xcex3(2f0,z)xe2x80x83xe2x80x83(3)
The constant xe2x80x9c2xe2x80x9d in the exponent of the attenuating factor in the equation (3) is provided corresponding to the roundtrip propagation.
The frequency-dependent attenuation characteristic xcex1 of the body tissue is generally linear, and therefore satisfies the following equation.
xcex1(2f0,z)=2xc2x7xcex1(f0,z)xe2x80x83xe2x80x83(4)
By dividing the equation (1) by the equation (3), and using the equation (4), the following equation in which the influence of the attenuating characteristic xcex1 and the scattering characteristic xcex3 is eliminated can be obtained.
A2(z)/AII(z)=(P0(f0)/A0(2f0))xc2x7∫h(z)dzxe2x80x83xe2x80x83(5)
Upon differentiating the above equation by the distance z, h(z) can be given by the following equation.
h(z)=d{A2(z)/AII(z)}/dzxc2x7{A0(2f0)/P0(f0)}xe2x80x83xe2x80x83(6)
Because P0(f0) and A0(2f0) are the intensity and the amplitude of the transmission and are known, h(z) reflecting the non-linear parameter (B/A) can be estimated from the equation (6) using A2(z) and AII(z).
In one method for obtaining the amplitude A2(z) of the secondary harmonic from the echo signal when the fundamental is transmitted, the band of the fundamental may be removed from the echo signal by using a band pass filter (BPF). However, using this method, the secondary harmonic component, being weak, cannot be accurately detected when the band of the fundamental and the band of the secondary harmonic overlap one another.
As a technique for solving the above problem, Kamakura et al. proposed a method in The Journal of the Acoustical Society of Japan, vol.46, No.10 (1990). In this method, two pulses both having the center frequency f0 which differ from one another only in the signs are transmitted, thereby allowing separation of the fundamental and the secondary harmonic in the time domain to be performed by simple addition or subtraction of the echo signals of the two transmissions. According to this method, by adding the echo signals, the two fundamental components opposite in polarity cancel out one another, and only the secondary harmonic component can be extracted. On the other hand, by subtracting the echo signals of the two transmissions, only the fundamental component can be extracted.
As described above, according to the conventional techniques, in order to extract the secondary harmonic component at high precision so as to calculate the evaluation value of the non-linear parameter from which the influence of the attenuating and scattering characteristics is removed, it is necessary to twice transmit fundamentals having the frequency f0 and differing polarity, and additionally transmit the fundamental having the frequency 2f0. In other words, the number of transmissions and receptions must undesirably be increased. When the number transmissions and receptions are increased with respect to one beam direction, the data rate becomes lowered. For that reason, when, for example, the non-linear parameter is visualized into an image, the frame rate is disadvantageously reduced. Further, error may be increased due to movement of the body tissue during a longer data acquisition time.
Moreover, because the main objective of the conventional art is the extraction and visualization of the evaluation values of the non-linear parameter, the influence of both the attenuating characteristic xcex1 and the scattering characteristic xcex3 within the body tissue is removed as described above. However, because the scattering characteristic xcex3 includes information concerning the body tissue structure, removing the scattering characteristic xcex3 results in de-emphasized expression of the tissue structure in an image. Depending on the purpose of the apparatus for diagnosis or the like, an image may be more useful when the image clearly displays, together with the evaluation values of the non-linear parameter, the body tissue structures that produce strong reflection and scattering echo, such as a muscle, a membrane or a valve. Using the above-described conventional technique, such a requirement cannot sufficiently be satisfied because the edges of the body tissue structures become blurred. To solve this problem, an arrangement may be devised to remove only the attenuating characteristic xcex1 from the amplitude A2 of the secondary harmonic by using the STC (sensitivity time control) function of the apparatus. However, adjustment of the STC is uniform with respect to an azimuth direction, i.e., the STC cannot be adjusted for each of the ultrasonic beams. For this reason, the influence of the attenuating characteristic xcex1 cannot sufficiently be removed, and improvement of the image quality is inhibited or limited.
The present invention was conceived in view of the above problems. An object of the present invention is to provide an ultrasonic diagnosis apparatus which can evaluate a non-linear parameter with high precision using a simpler structure, and which provides an image having a high quality based on the evaluation value of the non-linear parameter.
According to the present invention, there is provided an ultrasonic diagnosis apparatus comprising a transmitter for transmitting toward a living body a first transmission signal and a second transmission signal reverse in polarity to the first transmission signal, the first transmission signal containing a first fundamental component of a center frequency f0 and a second fundamental component of a center frequency 2f0; a receiver for outputting a first receiving signal corresponding to an echo of the first transmission signal and a second receiving signal corresponding to an echo of the second transmission signal; a sum signal generator for generating a sum signal based on addition of the first receiving signal and the second receiving signal; a difference signal generator for generating a difference signal based on subtraction between the first receiving signal and the second receiving signal; and an evaluation value calculator for calculating an evaluation value based on the sum signal and the difference signal.
According to the present invention, the evaluation value of a non-linear parameter or the like can be obtained by performing only twice each of transmission and reception of ultrasounds. In each of the transmissions, two fundamentals whose center frequencies are f0 and 2f0, respectively, are transmitted together. Between the first and second transmissions, the phases of the respective frequency components are shifted from one another by 180xc2x0. As a result, the first and second transmission signals have reverse polarity and cancel when added. In the transmitter, a transmitter for transmitting the first transmission signal and a transmitter for transmitting the second transmission signal may be provided separately, or both of the first and second transmission signals may be generated by a common structure. Both of the first and second transmission signals are basically transmitted to the same portion of a living body at different timings, and their respective echoes are received by the receiver. The sum signal generator adds the first receiving signal and the second receiving signal, and outputs, as the sum signal, the addition result or a signal corresponding to the addition result. Further, the difference signal generator performs subtraction between the first receiving signal and the second receiving signal, and outputs, as the difference signal, the subtraction result or a signal corresponding to the subtraction result. Each of the first receiving signal and the second receiving signal contains components, basically derived from each transmission signal, with reverse polarity, and components with identical polarity generated by interaction with the body. Accordingly, the components with reverse polarity are canceled out in the sum signal, and the sum signal basically includes a signal corresponding to a component generated by the non-linear interaction between the ultrasound and the medium. On the other hand, the components with identical polarity are canceled out in the difference signal, and the difference signal mainly comprises the component derived from the transmission signals. The sum signal and the difference signal are similarly influenced in the body by the frequency-dependent attenuation characteristic, the backscattering characteristic, and the like. Accordingly, by using the sum signal and the difference signal, data processing can be performed to, for example, remove the influence. The evaluation value calculator calculates the evaluation value based on the sum signal and the difference signal.
In one aspect of the present invention, the transmitter of the ultrasonic diagnosis apparatus includes a first fundamental generator for generating the first fundamental component, and a second fundamental generator for generating the second fundamental component.
In an ultrasonic diagnosis apparatus according to another aspect of the present invention, the difference signal generator extracts, as the difference signal, the second fundamental component included in the subtraction result between the first receiving signal and the second receiving signal.
The sum signal may include mainly the secondary harmonic component generated by the non-linear interaction between the first fundamental component and the body. That is, the sum signal is subjected to a frequency-dependent influence, such as attenuation, in accordance with the frequency 2f0. On the other hand, the subtraction result generates a signal corresponding to the sum of the first and second fundamental components included in the first and second transmission signals, respectively. According to the present invention, the second fundamental component among those components is extracted as the difference signal. Because the difference signal consisting of the extracted second fundamental component is located in the same band as the main component of the sum signal, the difference signal receives the similar influence of attenuation or the like within the body. Accordingly, by using the present invention, the influence of attenuation or the like within the body can readily be canceled out and removed from the sum signal.
In one aspect of the present invention, the difference signal generator of the ultrasonic diagnosis apparatus comprises a band pass filter which passes and extracts the second fundamental component included in the subtraction result.
In an ultrasonic diagnosis apparatus according to a further aspect of the present invention, the evaluation value calculator calculates an amplitude ratio of the sum signal and the difference signal, and then calculates the evaluation value based on the amplitude ratio.
The influence of attenuation or the like within the body is represented as a multiplication factor with respect to the signal. According to the present invention, by calculating the amplitude ratio of the sum signal to the difference signal, this multiplication factor can be canceled out, and an evaluation value without the influence of the multiplication factor can be calculated.
According to another aspect of the present invention, the ultrasonic diagnosis apparatus further comprises a detector for detecting the respective amplitude modulations of the sum signal and the difference signal. In addition, the evaluation value calculator calculates the evaluation value based on a detection signal output from the detector, the detection signal corresponding to each of the sum signal and the difference signal.
The processing for calculating the evaluation value is executed mainly by employing the amplitude information of the sum signal and the difference signal. According to the present invention, a change in the amplitude of the sum signal and difference signal vibrating at the frequency about 2f0 is extracted as the detection signal. The use of the detection signal facilitates the processing for calculating the evaluation value.
The ultrasonic diagnosis apparatus according to another aspect of the present invention further comprises a differentiator for calculating a ratio of change over time of the amplitude ratio.
According to the present invention, the change ratio of the amplitude ratio in the respective depths on a path of the ultrasound is obtained by calculating the ratio of change over time of the amplitude ratio. The evaluation value at the respective points on the ultrasound path can then be defined by using the change ratio of the amplitude ratio as the function of the depth.
In a preferred embodiment of the present invention, the differentiator of the ultrasonic diagnosis apparatus comprises a high pass filter.
An ultrasonic diagnosis apparatus according to a still further aspect of the present invention comprises an image generator for generating a tomographic image based on the evaluation value of a sectional plane of the body.
According to the present invention, the evaluation values at the respective points of a sectional plane of the body are obtained by scanning the body with the ultrasound transmitted from the transmitter. Visualizing the evaluation values as an image enables observers to readily perform diagnosis of a body based on the evaluation values.
Further, according to another aspect of the present invention, there is provided an ultrasonic diagnosis apparatus comprising a transmitter for transmitting toward a living body a first transmission signal and a second transmission signal reverse in polarity to the first transmission signal, the first transmission signal containing a first fundamental component of a center frequency f0 and a second fundamental component of a center frequency 2f0; a receiver for outputting a first receiving signal corresponding to an echo of the first transmission signal and a second receiving signal corresponding to an echo of the second transmission signal; a sum signal generator for generating a sum signal based on addition of the first receiving signal and the second receiving signal; a difference signal generator for generating a difference signal based on subtraction between the first receiving signal and the second receiving signal; an attenuation characteristic signal generator for generating, in accordance with the difference signal, an attenuation characteristic signal representative of the attenuation characteristic of the ultrasound corresponding to a depth; and a normalization circuit for normalizing the sum signal by using the attenuation characteristic signal, so as to output a normalization signal.
According to this aspect of the invention, the information pertaining to the non-linear interaction between the medium and the ultrasound is obtained via only twice of each of transmission and reception of ultrasounds, as in the above-described aspect. As was already described, the sum signal and the difference signal are similarly influenced within the body by the frequency-dependent attenuation characteristic and the backscattering characteristic. The attenuation characteristic signal generator generates the attenuation characteristic signal from the difference signal. The influence of the frequency-dependent attenuation characteristic of the ultrasound corresponding to a depth of the body present in the difference signal significantly remains in the attenuation characteristic signal, whereas the influence of the backscattering characteristic is partially-or completely removed. The normalization circuit normalizes the sum signal by using the attenuation characteristic signal to generate the normalization signal. The influence of the frequency-dependent attenuation characteristic present in the sum signal is removed from the normalization signal, whereas the influence of the backscattering characteristic remains in the normalization signal. In other words, the information on the non-linear interaction and the backscattering characteristic, which are found in the sum signal, remain in the normalization signal. Because this normalization signal includes the information on the backscattering characteristic, it is preferable for use in expression of the body tissue structure than is a signal including only the information on the non-linear interaction.
An ultrasonic diagnosis apparatus according to another aspect of the present invention further comprises a differentiator for differentiating the normalization signal to output a differentiation signal.
The component of the non-linear interaction included in the normalization signal corresponds to the integration of the non-linear interaction on a round-trip propagating path of the ultrasound up to the respective depths. According to the present invention, the normalization signal is differentiated with respect to the depthwise direction with the result that the obtained differentiation signal represents the intensity of the non-linear interaction in the respective depths on the path of the ultrasound.
In the ultrasonic diagnosis apparatus according to another aspect of the present invention, the attenuation characteristic signal generator suppresses a level fluctuation in the difference signal to generate the attenuation characteristic signal.
While the difference signal has a gradual macroscopic tendency for the signal from a deeper portion to become weaker in accordance with the attenuation characteristic of the ultrasound, level fluctuates, in a smaller distance scale, in accordance with the backscattering characteristic as influenced by the structure of the body tissue. The amplitude of the microscopic level fluctuation which may be caused by the backscattering characteristic can be relatively large. According to the present invention, the microscopic level fluctuation is suppressed to reduce the influence of the backscattering characteristic included in the difference signal, so as to generate the attenuation characteristic signal.
In the ultrasonic diagnosis apparatus according to the present invention, the attenuation characteristic signal generator clips a level fluctuation of the difference signal at a predetermined level and smoothes the difference signal after clipping, so as to generate the attenuation characteristic signal.
According to the present invention, a predetermined level of upper threshold, lower threshold, or both thresholds are designated with respect to level fluctuation of the difference signal. When the difference signal generates a level fluctuation that exceeds the threshold, a clipping process is executed to replace the signal value by the threshold value. Further, the clipped signal is smoothed to smooth out the microscopic level fluctuation. Through the clipping and smoothing process, the level fluctuation of the difference signal is suppressed, thereby allowing generation of the attenuation characteristic signal including a reduced influence of the backscattering characteristic.
In the ultrasonic diagnosis apparatus according to a preferred embodiment of the present invention, the above-noted predetermined level is a function of the depth. The difference signal has a tendency to reduce in level from a shallower portion of the body toward a deeper portion thereof due to the influence of the attenuation characteristic. Making use of this tendency, the predetermined level, which serves as the threshold of the difference signal, can be designated as a function of the depth so as to produce the attenuation characteristic signal in which the fluctuation due to the backscattering characteristic is favorably removed but the attenuation characteristic remains.
According to the present invention, there is provided an ultrasonic diagnosis apparatus comprising a transmitter for transmitting toward a living body a first transmission signal and a second transmission signal reverse in polarity to the first transmission signal, the first transmission signal containing a first fundamental component of a center frequency f0 and a second fundamental component of a center frequency 2f0; a receiver for outputting a first receiving signal corresponding to an echo of the first transmission signal and a second receiving signal corresponding to an echo of the second transmission signal; a sum signal generator for generating a sum signal based on addition of the first receiving signal and the second receiving signal; a difference signal generator for generating a difference signal based on subtraction between the first receiving signal and the second receiving signal; a first logarithmic converter for logarithmically converting the sum signal to output a logarithmic format sum signal; a second logarithmic converter for logarithmically converting the difference signal to output a logarithmic format difference signal; an attenuation characteristic signal generator for generating, in accordance with the logarithmic format difference signal, a logarithmic format attenuation characteristic signal representative of the attenuation characteristic of the ultrasound corresponding to a depth; and a normalization circuit for normalizing the logarithmic format sum signal by using the logarithmic format attenuation characteristic signal, so as to output a logarithmic format normalization signal.
According to the present invention, after the difference signal is logarithmically converted, the attenuation characteristic signal including a reduced influence of the backscattering characteristic is generated. The logarithmic format sum signal is then normalized using the logarithmic format attenuation characteristic signal.
According to another aspect of the present invention, there is provided an ultrasonic diagnosis apparatus comprising a transmitter for transmitting toward a living body a first transmission signal and a second transmission signal reverse in polarity to the first transmission signal, the first transmission signal containing a first fundamental component of a center frequency f0 and a second fundamental component of a center frequency 2f0; a receiver for outputting a first receiving signal corresponding to an echo of the first transmission signal and a second receiving signal corresponding to an echo of the second transmission signal; a sum signal generator for generating a sum signal based on addition of the first receiving signal and the second receiving signal; a difference signal generator for generating a difference signal based on subtraction between the first receiving signal and the second receiving signal; a first logarithmic converter for logarithmically converting the sum signal to output a logarithmic format sum signal; an attenuation characteristic signal generator for generating, in accordance with the difference signal, an attenuation characteristic signal representative of the attenuation characteristic of the ultrasound corresponding to a depth; a second logarithmic converter for logarithmically converting the attenuation characteristic signal to output a logarithmic format attenuation characteristic signal; and a normalization circuit for normalizing the logarithmic format sum signal by using the logarithmic format attenuation characteristic signal, so as to output a logarithmic format normalization signal.
According to the present invention, after the attenuation characteristic signal including a reduced influence of the backscattering characteristic is generated from the difference signal, logarithmic conversion is conducted to generate the logarithmic format attenuation characteristic signal. The logarithmic format sum signal is then normalized using the logarithmic format attenuation characteristic signal.